The polo related kinases are essential for cell cycle regulation from yeast to mammals. While yeast have only a single polo kinase, mammals have three related enzymes termed polo-like kinases (Plk) 1-3. Plk1 is activated at the G2M transition and its activity is required for the successful completion of mitosis. Over-expression of Plk1 in tissue culture leads to mitotic abnormalities and oncogenic transformation. Plk1 is over-expressed in multiple types of human tumors and may be used as a prognostic indicator. Plk1 is also re-expressed in Alzheimer's neurons whereas it is only normally expressed at detectable levels in proliferation cells. Both Plk1 and Plk3 are responsive in cultured cells to treatments that damage DNA and are implicated in checkpoint regulation. Plk1 is a member of several distinct multi-protein complexes and is responsible for regulating several mitotic events and its deregulation may lead to human disease. The focus of the Cell Cycle Section of The Laboratory of Cancer Prevention is on characterizing the roles of Plks during the cell cycle and in disease. Our present work involves studying the regulation of Plk activity and identifying proteins that interact with or are targets of Plk mediated phosphorylation. We have recently identified both Dia2 and Ran as Plk1 interacting proteins and have shown that Plk1 phosphorylates Ran. Over-expression of a mutant that mimics phosphorylated Ran disrupts Ran localization in interphase cells and causes an increased frequency of abnormal divisions in mitotic cells. While Plk1 does not phosphorylate Dia2, the two proteins are associated and co-localize at distinct regions of both interphase and mitotic cells. Kinase dead mutant Plk1 is able to block Dia2 induced stress fiber formation and may also affect Dia2 functions during cytokinesis.